The cytolytic action of complement is mediated by complement proteins C5, C6, C7, C8, and C9. Following activation of C5 on a natural or artificial bilayer membrane by the classical or alternative pathway, these complement proteins become assembled into a macromolecular structure, part of which is inserted into the lipid bilayer of the membrane in such a fashion as to form a trans-membrane channel. In the case of erythrocytes, as well as E. coli, it has been shown that a single complement channel kills the cell. Since strong evidence supporting this channel concept has been developed during the past six years, we are now directing our attention to the structure and mode of assembly of the channels. Specifically, we plan to study the size distribution and open time of the complement channels, the partition of complement protein and membrane lipids between bilayers and lipoprotein complexes, and the kinetics of insertion of complement protein and subsequent assembly within the membrane.